Energy Secretary Donald P. Hodel has abolished the office of assistant secretary for conservation and renewable energy, a move that is likely to reignite a feud between Congress and the Reagan administration.

In a memorandum that circulated in the department this month outlining an "organizational realignment," Hodel said the conservation job would be scrapped in favor of expanded responsibilities for Undersecretary Pat Collins. According to the memo, Collins will be given two additional deputies to run the day-to-day operations of DOE's energy-saving and renewable-energy research programs.

In a followup letter to DOE employes, Hodel said the change would give the programs "heightened policy visibility" and assure "that renewables and conservation achieve their appropriate positions in the nation's balanced and mixed energy resource base."

But three House Democrats have questioned the legality of the move, noting that the reorganization leaves the department with only seven assistant secretaries instead of the eight called for in the 1977 law that established the department.

In a letter last week to Hodel, House Energy and Commerce Committee Chairman John D. Dingell (D-Mich.) and two of his subcommittee chairmen, Richard L. Ottinger (D-N.Y.) and Philip R. Sharp (D-Ind.), also complained that news of the reorganization came to them through the Washington grapevine rather than from the DOE.

"That is hardly good congressional relations," they noted stiffly, adding that the abruptness of the change "raises doubts about the purpose of the transfers."

The White House has repeatedly sought to abolish most of DOE's conservation programs, arguing that saving energy was a job better left to the private sector. Against that policy background, the latest move has been widely interpreted as a signal that the abolition effort will be stepped up in President Reagan's second term.

Noting that the DOE had not yet worked out all the details of the move, the congressmen said they were concerned that "uncertainty and speculation" would "cause many valuable and experienced people . . . to begin to look for positions elsewhere."

In practical terms, however, Hodel's action is perhaps more symbolic than real, because he has essentially removed an empty chair. The conservation job has been vacant since Reagan's first appointee, Joseph J. Tribble, resigned more than a year ago.

Tribble's tenure was marked by frequent acrimonious exchanges on Capitol Hill, where such programs as weatherization aid for low-income households enjoy broad bipartisan support. The simmering differences came to a boiling point in late 1982, when Tribble fired the department's leading conservation expert after she turned down a transfer to Colorado.

DOE spokesman Jim Merna denied yesterday that the reorganization meant a lower priority for conservation programs. "We are not de-emphasizing conservation," he said. "We're accelerating it to the level of the undersecretary."

Dingell and his colleagues were skeptical of that argument, contending that the transfer would put control of the programs into the hands of two DOE employes who are not subject to Senate confirmation.

"That is not desirable," they wrote.

Congressional sources said they believed that Hodel wanted to use the assistant secretary slot to establish a high-level division of power marketing.

Merna acknowledged that the assistant secretaryship "could be re-established" with different responsibilities. "We know there'll be more changes," he said.